Control valves of the three-way and four-way types typically use machined spools which are axially movable in a housing to control fluid flow along paths to and from a work-producing device. Such a device may be a fluid piston and cylinder assembly having a piston rod as the work-producing component. Disclosures of servocontrol values of this general type are set forth in the following U.S. Pat. Nos. 3,580,281; 3,878,765; 4,083,381, 4,516,604; and 4,523,513.
Control valves using shiftable spools are extremely expensive to manufacture because of the many machining steps which are required. The spools must be subjected to honing, lapping and fluid edge grinding, and material for such a spool must be a high grade, heat treated tool steel. Moreover, in use, a spool valve is in sliding contact with the metallic body having the bore in which the spool valve moves. This metal-on-metal contact gives rise to friction which militates against fast response time and precision movement of the spool valve. Also, the metal-to-metal contact over time causes wear of the spool valves which reduces the precision of the fluid edge tolerances, thereby giving rise to errors in the operation of the spool valves.
Because of the foregoing problems, there is a continued need for improvements in control valves, and the present invention is directed to a control valve assembly which avoids the problems mentioned above with respect to conventional control valves with machined spools.